The present invention relates to monopulse direction finding (DF) radar systems and, more particularly, to a method and apparatus for electrically scanning an antenna array in a monopulse direction finding radar system to provide omniazimuthal coverage and unambiguous angle of arrival information.
Monopulse direction finding radar systems are well known in the art. Such systems use a simultaneous lobing technique wherein a pair of physically separated or offset overlapping antenna beams are utilized at the same time, instead of a single antenna beam which is monitored on a time shared basis. The overlapping antenna beams may be generated by two physically separate antennas or a single lens antenna illuminated by two adjacent feeds. Angle of arrival information is determined by measuring the relative phase or the relative amplitude of a single echo pulse received on each of the beams. The use of a monopulse system is highly desirable in certain applications, particularly in the electronic warfare art, because such a system is immune to pulse-to-pulse amplitude variations between received signals caused by scanning and/or propagation effects.
Scanning of the azimuth may be achieved in a monopulse system by a mechanically rotating pair of narrow beamwidth antennas. Ideally, the overlapping antenna beams would have a field of view limited to the direction in which same are pointed at any instant in time. As a practical matter, however, this is not the case because the antennas also have backlobes. A backlobe is a response to a signal in the direction which is the reverse or opposite of the direction in which the antenna is pointed. Backlobe responses are usually weaker than the response to a signal in the direction in which the antenna is pointed (forelobe response). However, sometimes the presence of a backlobe response makes an unambiguous angle of arrival determination difficult because it cannot be determined whether the received echo signal represents a strong signal picked up on a weak backlobe, or a weak signal picked up on the desired forelobe.
When the antennas are rotated by a mechanical drive, a two-channel rotary joint is required. However, rotary joints and other interconnections necessary in this instance often have narrow frequency responses, usually limited to an octave. Thus, wideband multi-octave antennas, covering a wide frequency reception range, cannot be readily used in mechanically driven scanning systems.
Two separate techniques have been utilized to eliminate the ambiguity in the angle of arrival determination and increase the range of frequency response in monopulse DF radar systems. The first technique combines a fixed omniazimuthal antenna and a rotating narrow beamwidth antenna, each connected to a different channel in a dual channel receiver. The gains of the channels are adjusted such that the response on the channel receiving the output of the omniazimuthal antenna is greater than the weakest backlobe of the narrow beamwidth antenna. A received signal to be processed to obtain the angle of arrival information is deemed acceptable only if the output of the channel connected to the narrow beamwidth antenna is greater than the output of the channel connected to the omniazimuthal antenna. In this manner, backlobe responses are inhibited.
This technique will theoretically permit unambiguous angle of arrival determinations. However, as a practical matter, the physical separation and differences in the characteristics between the two antennas causes unwanted variations in the channel responses. These variations in channel responses make the above method of backlobe suppression subject to error. Errors are present because it cannot be guaranteed that the output of the channel connected to the omniazimuthal antenna will always be less than the forelobe response, but greater than the backlobe response, of the channel connected to the narrow beamwidth antenna, for a signal in the direction in which the narrow beamwidth antenna is pointed. Thus, completely unambiguous angle of arrival determinations cannot be achieved with this technique.
The second technique for permitting unambiguous angle of arrival determinations utilizes four 90.degree. beamwidth stationary spiral antennas, geographically oriented to cover 360.degree., in four quadrants. Four reception channels are required, one for each antenna. Specific rules for signal acceptance are utilized for backlobe response suppression. When a signal is received on an antenna connected to one of the four associated receiver channels, it is determined if the signal is also present on either of the channels connected to antennas adjacent to the first antenna. If the signal is present on a channel connected to an adjacent antenna and is less in strength than the first, the signal is accepted and the monopulse ratio of strongest to adjacent next strongest is formed. However, if the signal is present on the antenna channel oriented in the opposite direction, the strength of the two signals is compared and since the channels are not adjacent, the channel with the strongest signal thereon is considered to be the desired channel. The adjacent next strongest signal is then sought to form the monopulse ratio. In this instance, the next strongest signal may be suppressed and the signal on the boresight channel processed, or both signals may be discarded and the determination made on the next pulse.
The four channel, four antenna system of signal detection is feasible when moderate DF accuracies are required. However, due to the wide beamwidths of the antennas and practical limitations on channel balancing in such a system, the degree of articulation (dB change per degree of azimuth coverage) and the sensitivity of the system are limited.
If better accuracy with omniazimuthal coverage is desired, without physical movement of the antennas, many more fixed antennas of narrower beamwidth, and an equal number of additional receiver channels, would be required. Narrower beamwidth antennas result in enhanced accuracy because of the increased dB per degree of azimuth coverage. Moreover, in a multi-channel system where there is a practical limit on the balance between the channels, system unbalance contributes less error for antennas of narrower beamwidth.
Thus, increasing the number of antennas and the number of receiver channels will permit unambiguous angle of arrival determination and improved accuracy, without decreasing the coverage or reducing frequency response. However, increasing the number of receiver channels is not a practical solution to the accuracy problem. The electronics required for a system having more than four balanced reception channels would be prohibitively expensive. Further, since DF systems are designed for use on mobile craft, most usually for airborne guidance systems and the like, the added space required by the extra electronics and the weight thereof would severely limit the applications for which such systems could be used.
It is, therefore, a prime object of the present invention to provide a method and apparatus for electrically scanning an antenna array in a monopulse DF radar system to provide omnidirectional coverage.
It is a second object of the present invention to provide a method and apparatus for electrically scanning an antenna array in a monopulse DF radar system wherein unambiguous angle of arrival information is obtained.
It is a third object of the present invention to provide a method and apparatus for electrically scanning an antenna array in a monopulse DF radar system wherein enhanced accuracy is achieved.
It is a fourth object of the present invention to provide a method and apparatus for electrically scanning an antenna array in a monopulse DF radar system wherein omniazimuthal coverage is achieved without mechanical rotation of antennas.
It is a fifth object of the present invention to provide a method and apparatus for electrically scanning an antenna array in a monopulse DF radar system wherein reception coverage is achieved over a multi-octave frequency range.
It is a sixth object of the present invention to provide a method and apparatus for electrically scanning an antenna array in a monopulse DF radar system wherein backlobe suppression is reliably achieved.
It is another object of the present invention to provide a method and apparatus for electrically scanning an antenna array in a monopulse DF radar system wherein unity probability of intercept is achieved.
It is a still further object of the present invention to provide a method and apparatus for electrically scanning an antenna array in a monopulse DF radar system wherein omnidirectional coverage is achieved simultaneously with backlobe suppression.
It is a still further object of the present invention to provide a method and apparatus for electrically scanning an antenna array in a monopulse DF radar system wherein scanning of the antenna array is automatically controlled.
In accordance with the present invention, method and apparatus for electrically scanning the antenna array in a monopulse DF radar system is provided. The antenna array has a plurality of fixed narrow beamwidth antennas geographically oriented to provide omnidirectional coverage. From the antenna array, first and second sets of antennas are selected in sequence.
The first set of selected antennas includes a plurality of pairs of oppositely oriented antennas. The RF outputs of the antennas in each of the antenna pairs in the first selected set are combined in a power divider to obtain a combined output signal for each pair. The combined output signal for each pair is then connected to the input of a different one of several matched receiver channels. In each channel, the combined output signal connected thereto is filtered, detected and logarithmically amplified.
The logarithmic outputs from each receiver are tested to determine if adjacent antennas have received the strongest and next strongest signals. If they have, a monopulse ratio formed by subtracting the logarithmic outputs from the receiver channels connected to the antenna pairs receiving the strongest and next strongest signals, is accepted for further processing. This ratio, however, cannot alone determine the angle of arrival because each antenna pair in the first selected set, and, thus, the antenna pairs generating the strongest and next strongest signals, includes antennas which are oppositely oriented, that is, offset by 180.degree.. Thus, the monopulse ratio contains an 180.degree. ambiguity.
In order to resolve this ambiguity, a second set of antennas is selected to include the two antennas which comprised the pair in the first selected set which generated the strongest combined signal. Each antenna in the second selected set is connected to the input of a different receiver channel.
The logarithmic outputs of the receiver channels connected to the antennas of the second selected set are compared to determine which represents the forelobe response. The signal representing the backlobe response is suppressed. The quadrant in which the antenna producing the forelobe response is situated is noted.
When the ratio is converted into angle of arrival information, the information noted relating to the quadrant in which is situated the antenna demonstrating the forelobe response is used to resolve the ambiguity present in the previously formed monopulse ratio. In this manner, unambiguous angle of arrival information is obtained with omnidirectional coverage.
Selection of the antenna sets is achieved by an electronically controlled switching circuit. The switching circuit comprises first and second sets of interconnected electronically controllable switches. The first set includes eight single pole, double throw switches, each having an RF input and two RF outputs. The RF input of each switch in the first set is connected to a different one of the eight antennas. The RF outputs of the switches in the first set are operably connected to the inputs of a second set of switches comprising four single pole, triple throw switches. Each switch in the second set has three RF inputs and an RF output. The RF output of each switch in the second set is connected to the input of one of the receiver channels.
The switching circuit is electronically controlled by a scan control circuit which generates biasing signals to control the states of the switches. In this manner, the appropriate antennas for each set are selected.